Ion exchange membrane for alkali chloride electrolysis, and alkali chloride electrolysis apparatus

ABSTRACT

To provide an ion exchange membrane for alkali chloride electrolysis which has a low membrane resistance and which is capable of reducing the electrolysis voltage during the alkali chloride electrolysis, while increasing the membrane strength. 
     An ion exchange membrane  1  for alkali chloride electrolysis wherein a reinforcing material  20  obtained by weaving with reinforcing yarns  22  and sacrificial yarns  24  is embedded in a fluoropolymer having ion exchange groups, the ion exchange membrane  1  comprises elution holes ( 28 ) formed by eluting at least a portion of a material of the sacrificial yarns  24 , and in a cross section perpendicular to the length direction of the yarns, the total area (S) obtained by adding the cross-sectional area of an elution hole  28  and the cross-sectional area of a sacrificial yarn  24  remaining in the elution hole  28  is from 500 to 1,200 μm 2 , and the number (n) of elution holes  28  between adjacent reinforcing yarns  22  is at least 10.

TECHNICAL FIELD

The present invention relates to an ion exchange membrane for alkalichloride electrolysis, and an alkali chloride electrolysis apparatus.

BACKGROUND ART

As an ion exchange membrane to be used in an alkali chlorideelectrolysis method for producing an alkali hydroxide and chlorine byelectrolyzing an alkali chloride aqueous solution such as seawater, anelectrolyte membrane made of a fluoropolymer having ion-exchange groups(carboxylic acid functional groups, sulfonic acid functional groups,etc.) is known.

In order to maintain the mechanical strength and dimensional stability,the electrolyte membrane is usually reinforced by a reinforcing fabricmade of reinforcing yarns (such as polytetrafluoroethylene (hereinafterreferred to as PTFE.) yarns). However, with an ion-exchange membranehaving a reinforcing fabric made of PTFE yarns, etc., the membraneresistance tends to be high, and the electrolysis voltage tends toincrease.

Therefore, a method using a reinforcing fabric obtained by interweavingreinforcing yarns of PTFE, etc. and sacrificial yarns soluble in analkaline aqueous solution, such as polyethylene terephthalate(hereinafter referred to as PET) yarns, has been proposed (e.g. PatentDocument 1).

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP-A-2004-43594

DISCLOSURE OF INVENTION Technical Problem

In the ion exchange membrane using the above-mentioned reinforcingfabric comprising the reinforcing yarns and the sacrificial yarns, inorder to enhance the membrane strength, it is important to furthernarrow the spacing between reinforcing yarns in the reinforcing fabric.However, if the spacing between reinforcing yarns is narrowed, themembrane resistance will increase, and the electrolysis voltage becomeshigh. Therefore, it is difficult to reduce the electrolysis voltage,while increasing the membrane strength of the ion exchange membrane.

It is an object of the present invention to provide an ion exchangemembrane for alkali chloride electrolysis which has a low membraneresistance and which is capable of reducing the electrolysis voltageduring the alkali chloride electrolysis, even if the spacing betweenreinforcing yarns is made narrow to increase the membrane strength, andan alkali chloride electrolysis apparatus using such an ion exchangemembrane for alkali chloride electrolysis.

Solution to Problem

The present invention has the following constructions.

[1] An ion exchange membrane for alkali chloride electrolysis comprisinga fluoropolymer having ion exchange groups, a reinforcing materialembedded in the fluoropolymer and formed of reinforcing yarns andoptionally contained sacrificial yarns, and elution holes of thesacrificial yarns present between the reinforcing yarns, characterizedin that

in a cross section perpendicular to the length direction of thereinforcing yarns forming the reinforcing material, the total area (S)obtained by adding the cross-sectional area of an elution hole and thecross-sectional area of a sacrificial yarn remaining in the elution holeis from 500 to 1,200 μm², and

the number (n) of elution holes between adjacent reinforcing yarns is atleast 10.

[2] The ion exchange membrane for alkali chloride electrolysis accordingto the above [1], wherein in a cross section perpendicular to the lengthdirection of the reinforcing yarns forming the reinforcing material, theaverage distance (d1) from the center of a reinforcing yarn to thecenter of the adjacent reinforcing yarn is from 750 to 1,500 μm.[3] The ion exchange membrane for alkali chloride electrolysis accordingto the above [1] or [2], wherein a relationship is established tosatisfy the following formula (1) in a cross section perpendicular tothe length direction of the reinforcing yarns:0.5≤{d2/d1×(n+1)}≤1.5  (1)provided that the symbols in the formula (1) have the followingmeanings,

d1: the average distance from the center of a reinforcing yarn to thecenter of the adjacent reinforcing yarn,

d2: the average distance from the center of an elution hole to thecenter of the adjacent elution hole,

n: the number of elution holes between adjacent reinforcing yarns.

[4] The ion exchange membrane for alkali chloride electrolysis accordingto the above [3], wherein a relationship is established to satisfy thefollowing formula (1′) at all measurement points measured to determinethe average distance (d1) and the average distance (d2) in a crosssection perpendicular to the length direction of the reinforcing yarns:0.5≤{d2′/d1×(n+1)}≤1.5  (1′)provided that the symbols in the formula (1′) have the followingmeanings,

d2′: the distance from the center of an elution hole to the center ofthe adjacent elution hole,

d1 and n: the same as above.

[5] The ion exchange membrane for alkali chloride electrolysis accordingto any one of the above [1] to [4], wherein a relationship isestablished to satisfy the following formula (2) in a cross sectionperpendicular to the length direction of the reinforcing yarns:1.0≤{d3/d1×(n+1)}≤2.0  (2)provided that the symbols in the formula (2) have the followingmeanings,

d3: the average distance from the center of a reinforcing yarn to thecenter of the adjacent elution hole,

d1 and n: the same as above.

[6] The ion exchange membrane for alkali chloride electrolysis accordingto the above [5], wherein a relationship is established to satisfy thefollowing formula (2′) at all measurement points measured to determinethe average distance (d1) and the average distance (d3) in a crosssection perpendicular to the length direction of the reinforcing yarns:1.0≤{d3′/d1×(n+1)}≤2.0  (2′)provided that the symbols in the formula (2′) have the followingmeanings,

d3′: the distance from the center of an elution hole to the center ofthe adjacent elution hole,

d1 and n: the same as above.

[7] The ion exchange membrane for alkali chloride electrolysis accordingto any one of the above [1] to [6], wherein the widths of saidreinforcing yarns in a cross section perpendicular to the lengthdirection of the reinforcing yarns are from 70 to 160 μm.[8] The ion exchange membrane for alkali chloride electrolysis accordingto any one of the above [1] to [7], wherein the fluoropolymer having ionexchange groups includes a fluoropolymer having carboxylic acidfunctional groups and a fluoropolymer having sulfonic acid functionalgroups, and the reinforcing material is embedded in the fluoropolymerhaving sulfonic acid functional groups.[9] An alkali chloride electrolysis apparatus comprising an electrolyticcell provided with a cathode and an anode, and an ion exchange membranefor alkali chloride electrolysis as defined in any one of the above [1]to [7] partitioning a cathode chamber on the cathode side and an anodechamber on the anode side in the electrolytic cell.[10] A method for producing an ion exchange membrane for alkali chlorideelectrolysis, which comprises obtaining a reinforcing precursor membranehaving a reinforcing fabric composed of reinforcing yarns andsacrificial yarns, embedded in a precursor membrane comprising afluoropolymer having groups convertible to ion exchange groups, and thencontacting the reinforcing precursor membrane to an alkaline aqueoussolution to convert the groups convertible to ion exchange groups, toion exchange groups, and at the same time to elute at least a portion ofthe sacrificial yarns in the reinforcing fabric, thereby to obtain anion exchange membrane comprising a fluoropolymer having ion exchangegroups, a reinforcing material having at least a portion of thesacrificial yarns in the reinforcing fabric eluted, and elution holes,characterized in that

in a cross section perpendicular to the length direction of thereinforcing yarns forming the reinforcing material in the ion exchangemembrane,

the total area (S) obtained by adding the cross-sectional area of anelution hole and the cross-sectional area of a sacrificial yarnremaining in the elution hole, is from 500 to 1,200 μm², and

the number (n) of elution holes between adjacent reinforcing yarns is atleast 10.

[11] The method for producing an ion exchange membrane for alkalichloride electrolysis according to the above [10], wherein in a crosssection perpendicular to the length direction of the reinforcing yarnsforming the reinforcing material, the average distance (d1) from thecenter of a reinforcing yarn to the center of the adjacent reinforcingyarn is from 750 to 1,500 μm.[12] The method for producing an ion exchange membrane for alkalichloride electrolysis according to the above [10] or [11], wherein arelationship is established to satisfy the following formula (1) in across section perpendicular to the length direction of the reinforcingyarns:0.5≤{d2/d1×(n+1)}≤1.5  (1)provided that the symbols in the formula (1) have the followingmeanings,

d1: the average distance from the center of a reinforcing yarn to thecenter of the adjacent reinforcing yarn,

d2: the average distance from the center of an elution hole to thecenter of the adjacent elution hole,

n: the number of elution holes between adjacent reinforcing yarns.

[13] The method for producing an ion exchange membrane for alkalichloride electrolysis according to the above [11], wherein arelationship is established to satisfy the following formula (1′) at allmeasurement points measured to determine the average distance (d2) in across section perpendicular to the length direction of the reinforcingyarns:0.5≤{d2′/d1×(n+1)}≤1.5  (1′)provided that the symbols in the formula (1′) have the followingmeanings,

d2′: the distance from the center of an elution hole to the center ofthe adjacent elution hole,

d1 and n: the same as above.

[14] The method for producing an ion exchange membrane for alkalichloride electrolysis according to any one of the above [10] to [12],wherein a relationship is established to satisfy the following formula(2) in a cross section perpendicular to the length direction of thereinforcing yarns:1.0≤{d3/d1×(n+1)}≤2.0  (2)provided that the symbols in the formula (2) have the followingmeanings,

d3: the average distance from the center of a reinforcing yarn to thecenter of the adjacent elution hole or sacrificial yarn,

d1 and n: the same as above.

[15] The method for producing an ion exchange membrane for alkalichloride electrolysis according to the above [14], wherein arelationship is established to satisfy the following formula (2′) at allmeasurement points measured to determine the average distance (d3) in across section perpendicular to the length direction of the reinforcingyarns:1.0≤{d3′/d1×(n+1)}≤2.0  (2′)provided that the symbols in the formula (2′) have the followingmeanings,

d3′: the distance from the center of a reinforcing yarn to the center ofthe adjacent elution hole,

d1 and n: the same as above.

[16] The method for producing an ion exchange membrane for alkalichloride electrolysis according to any one of the above [10] to [15],wherein the widths of said reinforcing yarns in a cross sectionperpendicular to the length direction of the reinforcing yarns are from70 to 160 μm.[17] A method for producing an alkaline chloride electrolysis apparatus,characterized by obtaining an ion exchange membrane for alkali chlorideelectrolysis by the method as defined in any one of the above [10] to[16], and then disposing the ion exchange membrane as an electrolytemembrane partitioning a cathode chamber on the cathode side and an anodechamber on the anode side in the electrolytic cell.

Advantageous Effects of Invention

The ion exchange membrane for alkali chloride electrolysis of thepresent invention has low membrane resistance and is capable of reducingthe electrolysis voltage during the alkaline chloride electrolysis evenif the spacing between reinforcing yarns is made narrow to increase themembrane strength.

The alkali chloride electrolysis apparatus of the present invention hasan ion-exchange membrane for alkali chloride electrolysis having a highmembrane strength, and the membrane resistance is low, and theelectrolysis voltage at the time of alkali chloride electrolysis is low.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing one example of theion exchange membrane for alkali chloride electrolysis of the presentinvention.

FIG. 2 is a schematic enlarged cross-sectional view of the vicinity ofthe surface of the layer (S) in the ion exchange membrane of alkalichloride electrolysis in FIG. 1.

FIG. 3 is a schematic diagram showing an example of the alkali chlorideelectrolysis apparatus of the present invention.

DESCRIPTION OF EMBODIMENTS

The following definitions of terms apply throughout the presentspecification including claims.

An “ion exchange group” is a group wherein at least a portion of ionscontained therein may be exchanged for another ion.

A “carboxylic acid functional group” means a carboxylic acid group(—COOH) or —COOM¹ (wherein M¹ is an alkali metal or a quaternaryammonium base).

A “sulfonic acid functional group” means a sulfonic acid group (—SO₃H)or —SO₃M² (wherein M² is an alkali metal or a quaternary ammonium base).

A “group convertible to an ion exchange group” means a group which canbe converted to an ion exchange group by a known treatment such ashydrolysis treatment, acid form treatment, etc.

A “group convertible to a carboxylic acid functional group” is a groupwhich can be converted to a carboxylic acid functional group by a knowntreatment such as hydrolysis treatment, acid form treatment, etc.

A “group convertible to a sulfonic acid functional group” is a groupwhich can be converted to a sulfonic acid functional group by a knowntreatment such as hydrolysis treatment, acid form treatment, etc.

A “fluoropolymer” means a polymer compound having fluorine atom(s) inthe molecule.

A “perfluorocarbon polymer” means a polymer wherein all of hydrogenatoms bonded to carbon atoms in the polymer are substituted by fluorineatoms. Some of the fluorine atoms in the perfluorocarbon polymer may besubstituted by chlorine atom(s) or bromine atom(s).

A “monomer” means a compound having a polymerization reactivecarbon-carbon double bond.

A “fluoromonomer” means a monomer having fluorine atom(s) in themolecule.

A “unit” means a moiety derived from a monomer present in a polymer toconstitute the polymer. A unit derived from a monomer, which is formedby addition polymerization of the monomer having a carbon-carbonunsaturated double bond, is a divalent unit formed by cleavage of theunsaturated double bond. Further, one having the structure of a certainunit chemically converted after polymer formation may also be referredto as a unit. In the following, in some cases, units derived from anindividual monomer may be represented by a name having the monomer'sname followed by “units”.

A “reinforcing fabric” means a fabric which is used as a raw materialfor a reinforcing material to improve the strength of an ion exchangemembrane. In the present specification, the “reinforcing fabric” isformed by weaving reinforcing yarns and sacrificial yarns. Reinforcingyarns and sacrificial yarns of the reinforcing fabric are woven as warpsand wefts, respectively, and these warps and wefts are orthogonal in thecase of a usual weaving method such as plain weaving.

A “reinforcing material” is a material which is used to improve thestrength of an ion exchange membrane, and means a material formed ofreinforcing yarns and optionally contained sacrificial yarns derivedfrom the reinforcing fabric, which is formed in the process forproducing an ion exchange membrane, by immersing a reinforcing precursormembrane made of a fluoropolymer having a reinforcing fabric embeddedtherein, in an alkaline aqueous solution (such as sodium hydroxideaqueous solution and potassium hydroxide aqueous solution), so that atleast a portion of the sacrificial yarns of the reinforcing fabric iseluted. When a portion of the sacrificial yarns is dissolved, thereinforcing material will be composed of the reinforcing yarns anddissolution residues of the sacrificial yarns, and when all of thesacrificial yarns are dissolved, the reinforcing yarns will be composedsolely of the reinforcing yarns. That is, the reinforcing material is amaterial formed of reinforcing yarns and optionally containedsacrificial yarns. The reinforcing material is embedded in an ionexchange membrane, and will be formed by immersing a reinforcingprecursor membrane made of a fluoropolymer having a reinforcing fabricembedded therein, in an alkaline aqueous solution.

Reinforcing yarns constituting the reinforcing material are derived froma reinforcing fabric and are thus comprised of warps and wefts. Thesewarps and wefts are usually orthogonal and are, respectively, present inparallel to the MD direction and the TD direction of the ion exchangemembrane.

Here, the MD (Machine Direction) is a direction in which, in theproduction of an ion exchange membrane, a precursor membrane, areinforcing precursor membrane and an ion exchange membrane areconveyed. The TD (Transverse Direction) is a direction perpendicular tothe MD direction.

A “sacrificial yarn” is a yarn constituting a reinforcing fabric andmeans a yarn made of a material which will elute in a sodium hydroxideaqueous solution (e.g. an aqueous solution with a concentration of 32mass %), when the reinforcing fabric is immersed therein. Onesacrificial yarn may be a monofilament consisting of one filament or maybe a multifilament composed of two or more filaments. In a case where asacrificial yarn is a multifilament, an assembly of two or morefilaments constitutes one sacrificial yarn. A sacrificial yarn will forman elution hole, as at least a portion thereof is eluted when areinforcing precursor membrane made of a fluoropolymer having areinforcing fabric embedded therein is immersed in an alkaline aqueoussolution such as sodium hydroxide aqueous solution and potassiumhydroxide aqueous solution. When a portion of a sacrificial yarn iseluted, the rest of the sacrificial yarn will remain undissolved in theelution hole. As the alkaline aqueous solution, for example, a sodiumhydroxide aqueous solution with a concentration of 32 mass %, may bementioned.

An “elution hole” means a hole to be formed as a result of elution ofone sacrificial yarn when the yarn is immersed in a sodium hydroxideaqueous solution (e.g. an aqueous solution with a concentration of 32mass %). In a case where the one sacrificial yarn is a monofilament, atleast a portion of the material of the monofilament will be eluted,whereby one hole will be formed in the ion exchange membrane. In a casewhere one sacrificial yarn is a multifilament, at least a portion of themultifilament will be eluted, whereby a collection of plural holes willbe formed in the ion exchange membrane, and this collection of pluralholes is one elution hole.

A “reinforcing yarn” is a yarn constituting a reinforcing fabric andmeans a yarn made of a material which will not be eluted even ifimmersed in a sodium hydroxide aqueous solution (e.g. an aqueoussolution with a concentration of 32 mass %). Even after immersing areinforcing precursor membrane made of a fluoropolymer having areinforcing fabric embedded therein, in an alkaline aqueous solution,whereby the sacrificial yarns are eluted from the reinforcing fabric, itremains undissolved as a yarn constituting the reinforcing material andmaintains the mechanical strength and dimensional stability of the ionexchange membrane for alkali chloride electrolysis.

A “center of a reinforcing yarn” means the center in the width directionof a reinforcing yarn in a cross section perpendicular to the lengthdirection of the reinforcing yarns of an ion exchange membrane.

A “center of an elution hole” means the center in the width direction ofan elution hole in a cross section perpendicular to the length directionof the reinforcing yarns of an ion exchange membrane. In a case where asacrificial yarn is a monofilament, the center of the sacrificial yarnbefore elution and the center of the elution hole will coincide witheach other. In a case where the sacrificial yarn is a multifilament, thecenter of an elution hole is meant for an intermediate point, in theabove-mentioned cross section, between the one end of holes and theother end of holes in the width direction.

An “aperture ratio” means a ratio of the area of the portion excludingthe reinforcing yarns, to the area in the surface direction of thereinforcing material.

A “reinforcing precursor membrane” means a membrane having a reinforcingfabric composed of reinforcing yarns and sacrificial yarns embedded in aprecursor membrane comprising a fluoropolymer having groups convertibleto ion exchange groups. It is preferred that two precursor membranescomprising a fluoropolymer having groups convertible to ion exchangegroups, are prepared, and a reinforcing fabric is laminated between thetwo precursor membranes.

A “precursor membrane” means a membrane comprising a fluoropolymerhaving groups convertible to ion exchange groups. It may be a membranecomposed of a single layer of a fluoropolymer having groups convertibleto ion exchange groups, or it may be a membrane composed of a pluralityof such layers.

Hereinafter, an ion exchange membrane of the present invention will bedescribed with reference to FIG. 1, but the present invention is notlimited to the contents of FIG. 1.

<Ion Exchange Membrane for Alkali Chloride Electrolysis>

The ion exchange membrane for alkali chloride electrolysis of thepresent invention comprises a fluoropolymer having ion exchange groups,a reinforcing material formed of reinforcing yarns and optionallycontained sacrificial yarns, present in a state of being embedded insaid fluoropolymer, and elution holes formed by elution of saidsacrificial yarns present between said reinforcing yarns.

FIG. 1 is a schematic cross-sectional view showing one example of theion exchange membrane for alkali chloride electrolysis of the presentinvention.

The ion exchange membrane 1 for alkali chloride electrolysis(hereinafter referred to as “ion exchange membrane 1”) is one having anelectrolyte membrane 10 comprising a fluoropolymer having ion exchangegroups, reinforced by a reinforcing fabric 20.

[Electrolyte Membrane]

The electrolyte membrane 10 is a laminate comprising a layer 12 made ofa fluoropolymer having carboxylic acid functional groups, as afunctional layer exhibiting high current efficiency, and a layer 14 aand layer 14 b made of a fluoropolymer having sulfonic acid functionalgroups, to maintain the mechanical strength.

(Layer 12 Made of Fluoropolymer Having Carboxylic Acid FunctionalGroups)

As the layer 12 (hereinafter referred to also as “layer (C)”) made of afluoropolymer having carboxylic acid functional groups (hereinafterreferred to also as “fluoropolymer (C)”), for example, a copolymercomprising units derived from a fluoromonomer having a carboxylic acidfunctional group and units derived from a fluoro-olefin, may bementioned.

The fluoropolymer (C) is obtained by converting groups convertible tocarboxylic acid functional groups in a fluoropolymer having the groupsconvertible to carboxylic acid functional groups as described later(hereinafter referred to also as “fluoropolymer (C′)”) in a step (b)described below, to carboxylic acid functional groups.

The layer (C) usually has a membrane form. The thickness of the layer(C) is preferably from 5 to 50 μm, more preferably from 10 to 35 μm.When the thickness of the layer (C) is at least the above lower limitvalue, a high current efficiency is likely to be expressed. Further, ina case where electrolysis of sodium chloride is conducted, it ispossible to reduce the amount of sodium chloride in sodium hydroxide asthe product. When the thickness of the layer (C) is at most the aboveupper limit value, the membrane resistance of the ion exchange membrane1 is suppressed to be low, and the electrolysis voltage is likely to below.

(Layer 14 a and Layer 14 b Made of Fluoropolymer Having Sulfonic AcidFunctional Groups)

As a layer 14 a (hereinafter referred to also as “layer (Sa)”) and alayer 14 b (hereinafter referred to also as “layer (Sb)”) made of afluoropolymer having sulfonic acid functional groups (hereinafterreferred to also as “fluoropolymer (S)”), a copolymer comprising unitsderived from a fluoromonomer having a sulfonic acid functional group andunits derived from a fluoro-olefin, may be mentioned.

The fluoropolymer (S) is obtained by converting, in a step (b) asdescribed below, groups convertible to sulfonic acid groups in afluoropolymer having the groups convertible to sulfonic acid functionalgroups as described later (hereinafter referred to also as“fluoropolymer (S′)”, to sulfonic acid functional groups.

FIG. 1 shows a state wherein a reinforcing material 20 is embeddedbetween the layer (Sa) and the layer (Sb) (hereinafter the layer (Sa)and layer (Sb) will be collectively referred to also as “layer (S)”). Ina laminated structure of two layers (Sa) and (Sb), the reinforcingmaterial 20 is embedded between the two layers and as a result, isembedded in the layer (S).

In FIG. 1, the layer (Sa) and the layer (Sb) are each a single layer,but each of them may be formed of plural layers.

The thickness of the layer (Sb) is preferably from 30 to 140 μm, morepreferably from 30 to 100 μm. When the thickness of the layer (Sb) is atleast the lower limit value, the mechanical strength of the ion exchangemembrane 1 will be sufficiently high. When the thickness of the layer(Sb) is at most the upper limit value, membrane resistance of the ionexchange membrane 1 will be suppressed to be sufficiently low, andincrease of the electrolysis voltage will be sufficiently suppressed.

The thickness of the layer (Sa) is preferably from 10 to 60 μm, morepreferably from 10 to 40 μm. When the thickness of the layer (Sa) is atleast the lower limit value, the reinforcing fabric 20 fits into theelectrolyte membrane 10 thereby to improve peeling resistance of thereinforcing fabric 20. Further, the reinforcing fabric 20 will not betoo close to the surface of the electrolyte membrane 10, wherebycracking is less likely to enter the surface of the electrolyte membrane10, and as a result, lowering of the mechanical strength can beprevented. When the thickness of the layer (Sa) is at most the upperlimit value, the membrane resistance of the ion exchange membrane 1 willbe suppressed to be sufficiently low, and increase of the electrolysisvoltage will be sufficiently suppressed.

(Reinforcing Material)

The reinforcing material 20 is a reinforcing material for reinforcingthe electrolyte membrane 10, and is a textile using a reinforcing fabricas a starting material and obtained by weaving with reinforcing yarns 22and sacrificial yarns 24.

In the ion exchange membrane of the present invention, in order tosecure the effect of the present invention, it is important that in across section perpendicular to the length direction of the reinforcingyarns forming the reinforcing material, the number of elution holes, andthe total area of the sum of the cross-sectional area of an elution holeand the cross-sectional area of a sacrificial yarn remaining undissolvedin the elution hole, are in specific ranges.

That is, in the ion exchange membrane of the present invention, in across section perpendicular to the length direction of the reinforcingyarns of the ion exchange membrane 1, the average distance (d1) from thecenter of a reinforcing yarn 22 to the center of the adjacentreinforcing yarn 22 is from 750 to 1,500 μm, preferably from 750 to1,300 μm, preferably from 800 to 1,200 μm, more preferably from 800 to1,100 μm, particularly preferably from 800 to 1,000 μm, moreparticularly preferably from 900 to 1,000 μm. When the average distance(d1) is within this range, it is possible to reduce the electrolysisvoltage during alkali chloride electrolysis, while increasing themembrane strength. When the average distance (d1) is at least the lowerlimit value, it is easy to reduce the electrolysis voltage during alkalichloride electrolysis. When the average distance (d1) is at most theupper limit value, it is easy to increase the membrane strength of theion exchange membrane 1.

The average distance (d1) means an average value of measured values ofthe distance from the center of a reinforcing yarn to the center of theadjacent reinforcing yarn in each of the MD cross section perpendicularto the length direction of the reinforcing yarns of the ion exchangemembrane (cross section cut perpendicular to the MD direction)perpendicular to the length direction of the warps of the reinforcingyarns and the TD cross section (cross section cut perpendicular to theTD direction). The average value in the present invention is an averagevalue of the measured values of distances measured at 10 points randomlyselected in each cross section, and the same applies to values otherthan d1.

In the present invention, it is preferred that the above-mentioneddistance from the center of a reinforcing yarn to the center of theadjacent reinforcing yarn in a cross section perpendicular to the lengthdirection of the reinforcing yarns of the ion exchange membrane, iswithin the above range at all the measurement points. It becomes therebyeasy to obtain the effect to reduce the electrolysis voltage during thealkali chloride electrolysis, while increasing the membrane strength.All measurement points are meant for all points measured at random inorder to calculate the average value. The same applies to values otherthan d1.

The density (the number of implantation) of reinforcing yarns 22 in thereinforcing fabric 20 is preferably from 15 to 34 yarns/inch, morepreferably from 18 to 34 yarns/inch, particularly preferably from 20 to34 yarns/inch, more particularly preferably 20 to 30 yarns/inch, mostparticularly preferably 24 to 30 yarns/inch. When the density of thereinforcing yarns 22 is at least the above lower limit value, themechanical strength as a reinforcing material will be sufficiently high.When the density of the reinforcing yarns 22 is at most the upper limitvalue, the membrane resistance of the ion exchange membrane 1 can besuppressed to be sufficiently low, and increase of the electrolysisvoltage can be sufficiently suppressed.

The density of sacrificial yarns 24 in the reinforcing fabric in orderto form the reinforcing material is preferably an even multiple of thedensity of the reinforcing yarns 22. Specifically, the density ofsacrificial yarns 24 is preferably 10 times or 12 times the density ofthe reinforcing yarns 22. When it is an odd multiple, the warp and weftreinforcing yarns 22 do not cross vertically alternately, so that afabric texture will not form after elution of the sacrificial yarns 24.

The total density of the reinforcing yarns 22 and the sacrificial yarns24 is preferably from 176 to 850 yarns/inch, more preferably from 198 to850 yarns/inch, particularly preferably from 220 to 750 yarns/inch, moreparticularly preferably 220 to 442 yarns/inch, most particularlypreferably 220 to 390 yarns/inch, from the viewpoint of less likelinessof misalignment.

The aperture ratio of the reinforcing material is preferably from 60 to90%, more preferably from 65 to 85%, further preferably from 70 to 85%,particularly preferably from 75 to 85%. When the aperture ratio of thereinforcing material is at least the lower limit value, the membraneresistance of the ion exchange membrane 1 can be suppressed to besufficiently low, and increase of the electrolysis voltage can besufficiently suppressed. When the aperture ratio of the reinforcingmaterial is at most the upper limit value, the mechanical strength as areinforcing material will be sufficiently high.

The aperture ratio of the reinforcing material can be determined from anoptical photomicrograph.

The thickness of the reinforcing material 20 is preferably from 40 to160 μm, more preferably from 60 to 150 μm, particularly preferably from70 to 140 μm, especially preferably from 80 to 130 μm. When thethickness of the reinforcing material 20 is at least the lower limitvalue, the mechanical strength as a reinforcing material will besufficiently high. When the thickness of the reinforcing member 20 is atmost the upper limit value, the thickness at the yarn intersections canbe suppressed, and it is possible to sufficiently suppress the influenceto raise the electrolysis voltage due to current shielding by thereinforcing material 20.

(Reinforcing Yarns)

The reinforcing yarns 22 are preferably ones having durability againsthigh temperature conditions in the alkali chloride electrolysis, as wellas against chlorine, sodium hypochlorite and sodium hydroxide. As thereinforcing yarns 22, from the viewpoint of mechanical strength, heatresistance and chemical resistance, yarns comprising a fluoropolymer arepreferred; yarns comprising a perfluorocarbon polymer are morepreferred; yarns comprising PTFE are further preferred; and PTFE yarnscomposed solely of PTFE are especially preferred.

The reinforcing yarns 22 may be monofilaments or may be multifilaments.In a case where the reinforcing yarns 22 are PTFE yarns, from such aviewpoint that spinning is easy, monofilaments are preferred, and tapeyarns obtained by slitting a PTFE film are more preferred.

The fineness of the reinforcing yarns 22 is preferably from 50 to 200denier, more preferably from 80 to 150 denier. When the fineness of thereinforcing yarns 22 is at least the lower limit value, the mechanicalstrength will be sufficiently high. When the fineness of the reinforcingyarns 22 is at most the upper limit value, the membrane resistance ofthe ion exchange membrane 1 can be suppressed sufficiently low, andincrease of the electrolysis voltage can be sufficiently suppressed.Further, the reinforcing yarns 22 will be less likely to be too close tothe surface of the electrolyte membrane 10, whereby cracking is lesslikely to enter the surface of the electrolyte membrane 10, and as aresult, lowering of the mechanical strength can be prevented.

The width of the reinforcing yarns 22 in a cross section to the lengthdirection of the reinforcing yarns, i.e., as viewed from a directionperpendicular to the reinforcing fabric forming the reinforcing material20, is from 70 to 160 μm, preferably from 90 to 150 μm, more preferablyfrom 100 to 130 μm. When the width of the reinforcing yarns 22 is atleast the above lower limit value, the membrane strength of the ionexchange membrane 1 tends to be high. When the width of the reinforcingyarns 22 is at most the upper limit value, it is easy to lower themembrane resistance of the ion exchange membrane 1, and easy to preventincrease in the electrolysis voltage.

(Sacrificial Yarns)

The sacrificial yarns 24 will form elution holes as at least a portionof the material is eluted in an alkaline aqueous solution in thefollowing step (i) for producing an ion exchange membrane. The ionexchange membrane obtained via the step (i) is then placed in anelectrolytic cell, and prior to the main operation of alkali chlorideelectrolysis, a conditioning operation of the following step (ii) iscarried out. Even in a case where sacrificial yarns 24 remainingundissolved in step (i) are present, in the step (ii), the majority orpreferably the entirety of the remaining material of the sacrificialyarns 24 is eluted and removed in the alkaline aqueous solution.

Here, the ion exchange membrane for alkali chloride electrolysis of thepresent invention is a membrane produced via the step (i) and disposedin an electrolytic cell in the step (ii), and is not a membrane afterthe conditioning operation in the step (ii).

Step (i): a step of immersing a reinforcing precursor membrane having areinforcing fabric embedded in a fluoropolymer having groups convertibleto ion exchange groups, in an alkaline aqueous solution, to convert thefluoropolymer having groups convertible to ion exchange groups, to afluoropolymer having ion exchange groups.

Step (ii): a step of disposing the ion exchange membrane obtained viathe step (i) in an electrolytic cell and carrying out a conditioningoperation before the main operation of alkali chloride electrolysis.

As the sacrificial yarns 24, preferred are yarns comprising at least onemember selected from the group consisting of PET, polybutyleneterephthalate (hereinafter referred to as PBT), polytrimethyleneterephthalate (hereinafter referred to as PTT), rayon and cellulose, andmore preferred are PET yarns composed solely of PET, PET/PBT yarnscomposed of a mixture of PET and PBT, PBT yarns composed solely of PBT,or PTT yarns composed solely of PTT.

As the sacrificial yarns 24, from the viewpoint of cost, PET yarns arepreferred. As the sacrificial yarns 24, from the viewpoint that it ispossible to obtain an ion-exchange membrane 1 which is hardly eluted inan alkaline aqueous solution during the step (i) and which hassufficiently high mechanical strength, PBT yarns or PTT yarns arepreferred, and PTT yarns are particularly preferred. As the sacrificialyarns 24, from the viewpoint of the balance between costs and themechanical strength of the ion exchange membrane 1, PET/PBT yarns arepreferred.

A sacrificial yarn 24 may be a monofilament formed from one filament 26or a multifilament having plural filaments 26 gathered as shown inFIG. 1. A monofilament is preferable from the viewpoint that yarnbreakage is not likely to occur during weaving. A multifilament ispreferable from the viewpoint of easiness of to being eluted in analkaline aqueous solution.

In a case where a sacrificial yarn 24 is a multifilament, the number offilaments per one sacrificial yarn 24 is preferably from 2 to 32, morepreferably from 2 to 16, further preferably from 2 to 8. When the numberof filaments in the multifilament is at least the above lower limitvalue, during the step (ii), a sacrificial yarn 24 will be easily elutedin the alkaline aqueous solution. When the number of filaments in themultifilament is at most the above upper limit value, during the step(i), such a sacrificial yarn will be hardly eluted in the alkalineaqueous solution, and part of the sacrificial yarn will remain, wherebyan ion-exchange membrane 1 having sufficiently high mechanical strengthwill be obtainable.

The fineness of the sacrificial yarns 24 is preferably from 6 to 14denier, more preferably from 7 to 12 denier, prior to elution of thesacrificial yarns 24 in the step (i). When the fineness of thesacrificial yarns 24 is at least the above lower limit value, themechanical strength will be sufficiently high, and, at the same time,weaving properties will be sufficiently high. When the fineness of thesacrificial yarns 24 is at most the above upper limit value, holes to beformed after the sacrificial yarns 24 are eluted, are less likely to betoo close to the surface of the electrolyte membrane 10, and cracking isless likely to occur at the surface of the electrolyte membrane 10,whereby deterioration in the mechanical strength can be avoided.

(Elution Holes)

The ion exchange membrane 1 has, in the layer (S) (14 a and 14 b) of theelectrolyte membrane 10, elution holes 28 formed by elution of at leasta portion of the material of the sacrificial yarns 24 during the step(i) and the step (ii). As shown in FIG. 1, in a case where a sacrificialyarn 24 is a monofilament composed of one filament, at least a portionof the material of the monofilament will be eluted to form an elutionportion comprising a collection of one hole. In a case where asacrificial yarn 24 is a multifilament, at least a portion of thematerial of the multifilament will be eluted to form an elution holecomprising a collection of plural holes. In a case where in the step(i), a portion of the sacrificial yarn 24 has remained without beingeluted, the remaining sacrifice yarn is present in the elution hole.

In the ion exchange membrane 1, it is preferred that even after the step(i), a portion of the sacrificial yarns 24 remains, and elution holes 28are formed around filaments 26 of the sacrificial yarns 24. Thereby,breakage such as cracking is less likely to occur in the ion exchangemembrane 1, at the time of handling the ion exchange membrane 1 afterthe production of the ion exchange membrane 1 and before theconditioning operation of the alkali chloride electrolysis, or at thetime of installation of the ion exchange membrane 1 in the electrolyticcell at the time of the conditioning operation.

Even if a portion of sacrificial yarns 24 remains after the step (i),during the step (ii), the sacrificial yarns 24 will be completely elutedin the alkaline aqueous solution and will be removed, so that at thetime of the main operation of the alkali chloride electrolysis using theion exchange membrane 1, they present no influence over the membraneresistance. After the installation of the ion exchange membrane 1 in theelectrolytic cell, there will be no large force to be exerted fromoutside to the ion exchange membrane 1, and therefore, even if thesacrificial yarns 24 have been completely eluted and removed in thealkaline aqueous solution, it is less likely that breakage such ascracking occurs in the ion exchange membrane.

Otherwise, in the present invention, all of the sacrificial yarns 24 maybe eluted in the entirety of sacrificial yarns 24 may be eluted duringthe step (i) so that elution holes 28 can form without remainingsacrificial yarns 24 prior to the step (ii).

In a cross section perpendicular to the length direction of thereinforcing yarns of the ion exchange membrane 1, the total area (S)obtained by adding the cross-sectional area of an elution hole 28 andthe cross-sectional area of a sacrificial yarn 24 in the elution hole28, is from 500 to 1,200 μm², preferably from 550 to 1,100 μm², morepreferably from 600 to 1,000 μm², further more preferably from 600 to900 μm², particularly preferably from 600 to 800 μm², per elution hole.In a case where the sacrificial yarns are completely eluted, the totalarea (S) becomes a cross-sectional area of only the elution hole. Thatis, the total area (S) obtained by adding the cross-sectional areas,becomes to be almost equal to the total area of the cross-sectional areaof one sacrificial yarn in the reinforcing fabric. In a case where asacrificial yarn is a monofilament, the total area (S) becomes thecross-sectional area of the elution hole formed from one filament, andin a case where a sacrificial yarn is a multifilament composed of two ormore filaments, the total area (S) becomes the sum of thecross-sectional areas of elution holes formed from the respectivefilaments constituting the multifilament.

When the total area (S) is at least the above lower limit value, it ispossible to prepare a reinforcing fabric without causing yarn breakageof the sacrificial yarns during weaving, and it is possible to reducethe electrolysis voltage during the alkali chloride electrolysis. Whenthe total area (S) is at most the above upper limit value, the holeformed after the sacrificial yarn 24 is eluted will not be too close tothe surface of the electrolyte membrane 10, whereby cracking is lesslikely to enter the surface of the electrolyte membrane 10, and as aresult, lowering of the mechanical strength can be prevented.

The total area (S) is measured by using an imaging software by observingthe cross section of the ion exchange membrane dried at 90° C. over 2hours, by an optical electron microscope.

In the present invention, in a cross section perpendicular to the lengthdirection of the reinforcing yarns forming a reinforcing material, thetotal area (S) is in the above range. A cross section perpendicular tothe length direction of the reinforcing yarns means at least one crosssection selected from a cross section (hereinafter referred to as “MDcross section”) cut perpendicular to the MD direction and a crosssection (hereinafter referred to as “TD cross section”) cutperpendicular to the TD direction, of the ion exchange membrane. Thatis, at least one of the total area (S) of the total area (S) in the TDcross section and the total area (S) in the MD cross section, is in theabove range.

Further, the MD cross-section of the ion-exchange membrane in thepresent invention is preferably a cross section which does not overlapwith the reinforcing yarns, the sacrificial yarns and the elution holes,disposed perpendicularly to the MD direction in the reinforcing materialembedded in the ion exchange membrane. The same applies to the TD crosssection.

The total area (S) in the cross section of the present invention is morepreferably such that the average value of the total area (S) in the MDcross section and the total area (S) in the TD cross section, is withinthe above range, further preferably such that both of the total area (S)in the MD cross section and the total area (S) in the TD cross sectionare within the above range.

The total area (S) in the MD cross section can be obtained by measuringthe total area (S) with respect to elution holes at 10 locationsrandomly selected in the MD cross section of an ion exchange, andobtaining the average value thereof. The total area (S) in the TD crosssection can be obtained also in the same manner.

In an ion exchange membrane, in a case where a sacrificial yarn iscompletely dissolved, the total area (S) is the cross-sectional area ofthe elution hole, and in a case where a sacrificial yarn remainingundissolved is present in an elution hole, the total area (S) is a totalarea obtained by adding the cross-sectional area of the elution hole andthe cross-sectional area of the sacrificial yarn remaining undissolved.

In a cross section perpendicular to the length direction of thereinforcing yarns of the ion exchange membrane 1, the number n ofelution holes 28 between adjacent reinforcing yarns 22 is at least 10,preferably from 10 to 20, particularly preferably from 10 to 12. Whenthe number n of elution holes 28 is at least 10, it is possible toreduce the electrolysis voltage during the alkaline chlorideelectrolysis, while increasing the membrane strength. Here, an elutionhole formed from one sacrificial yarn made of a multifilament is countedas 1.

The ion-exchange membrane of the present invention is a membrane inwhich the distances of reinforcing yarns and elution holes are withinthe above ranges, and the distance of elution holes is within the aboverange, but it is particularly preferably an ion exchange membrane havinga structure wherein the distances of reinforcing yarns and elutionholes, and the number of elution holes are in a relationship satisfyingcertain conditions, i.e. a structure wherein the distance of reinforcingyarns, and the number and distance of elution holes have a certainregularity.

That is, with respect to the distance of elution holes in the ionexchange membrane of the present invention, the average distance (d2)from the center of an elution hole 28 to the center of the adjacentelution hole 28 in a cross section perpendicular to the length directionof the reinforcing yarns of the ion-exchange membrane 1, preferablysatisfies the following relation (1), more preferably satisfies thefollowing relation (1-1), further preferably satisfies the followingformula (1-2), particularly preferably satisfies the following formula(1-3). Thereby, it becomes easy to obtain the effect of reducing theelectrolysis voltage during the alkali chloride electrolysis whileincreasing the membrane strength.0.5≤{d2/d1×(n+1)}≤1.5  (1)0.7≤{d2/d1×(n+1)}≤1.4  (1-1)0.8≤{d2/d1×(n+1)}≤1.2  (1-2)0.8≤{d2/d1×(n+1)}≤1.0  (1-3)wherein

d1: the average distance from the center of a reinforcing yarn to thecenter of the adjacent reinforcing yarn,

d2: the average distance from the center of an elution hole to thecenter of the adjacent elution hole,

n: number of elution holes between adjacent reinforcing yarns.

In the present invention, in a cross section perpendicular to the lengthdirection of the reinforcing yarns, it is preferred that the averagedistance (d1) and the average distance (d2) satisfy the relationships ofthe above formulas. A cross section perpendicular to the lengthdirection of the reinforcing yarns means at least one cross sectionselected from the MD cross section and the TD cross section of the ionexchange membrane. That is, the average distance (d1) and the averagedistance (d2) in at least one cross section selected from the MD crosssection and the TD cross section preferably satisfy the relationships ofthe above formulas.

In the present invention, it is preferred that an average value of theaverage distance (d1) in the MD cross section and the average distance(d1) in the TD cross section, and an average value of the averagedistance (d2) in the MD cross section and the average distance (d2) inthe TD cross section, satisfy the above formulas, and it is morepreferred that in both the MD cross section and the TD cross section,the average distance (d1) and the average distance (d2) satisfy therelationships of the above formulas.

The values of the average distance (d1) and the average distance (d2) inthe MD cross section, are obtainable by measuring, in the MD crosssection of the ion exchange membrane, the average distance (d1) and theaverage distance (d2), respectively, at 10 locations randomly selected,and obtaining the respective average values. The average distance (d1)and the average distance (d2) in the TD cross section are obtainable inthe same manner.

In the present invention, in a cross section perpendicular to the lengthdirection of the reinforcing yarns of the ion exchange membrane, all thedistance (d2′) from the center of an elution hole to the center of theadjacent elution hole, preferably satisfy the relation of the followingformula (1′), more preferably satisfies the relation of the followingformula (1′-1), further preferably satisfies the relation of thefollowing formula (1′-2), particularly preferably satisfies the relationof the following formula (1′-3), at all of the measurement pointsmeasured to determine the average distance (d2). Thereby, it becomeseasy to obtain the effect of reducing the electrolysis voltage duringthe alkali chloride electrolysis, while increasing the membranestrength.

Here, in the distance (d2′), all of the measurement points measured todetermine the average distance (d2) mean all of the measurement pointsmeasured to calculate the average distance (d2). Specifically, in the MDcross section or the TD cross section, all of the measurement pointsmean the measurement points at 10 locations measured to obtain theaverage distance (d2).0.5≤{d2′/d1×(n+1)}≤1.5  (1′)0.7≤{d2′/d1×(n+1)}≤1.4  (1′-1)0.8≤{d2′/d1×(n+1)}≤1.2  (1′-2)0.8≤{d2′/d1×(n+1)}≤1.0  (1′-3)provided that the symbols in the formula (1′) have the followingmeanings,

d2′: the distance from the center of an elution hole to the center ofthe adjacent elution hole,

d1 and n: the same as above.

Further, with respect to the distance between a reinforcing yarn and anelution hole in the ion exchange membrane of the present invention, in across section perpendicular to the length direction of the reinforcingyarns of the ion exchange membrane 1, an average distance (d3) from thecenter of a reinforcing yarn 22 to the center of the adjacent elutionhole 28 preferably satisfies the relation of the following formula (2),more preferably satisfies the relation of the following formula (2-1),further preferably satisfies the relation of the following formula(2-2). Thereby, it becomes easy to obtain the effect of reducing theelectrolysis voltage during the alkali chloride electrolysis, whileincreasing the membrane strength.1.0≤{d3/d1×(n+1)}≤2.0  (2)1.2≤{d3/d1×(n+1)}≤1.8  (2-1)1.5≤{d3/d1×(n+1)}≤1.8  (2-2)provided that the symbols in the formula (2) have the followingmeanings:

d3: the average distance from the center of a reinforcing yarn to thecenter of the adjacent elution hole.

d1 and n: the same as above.

Further, in the present invention, in a cross section perpendicular tothe length direction of the reinforcing yarns, the average distance (d1)and the average distance (d3) preferably satisfy the relations of theabove formulas. A cross section perpendicular to the length direction ofthe reinforcing yarns means at least one cross section of the MD crosssection and the TD cross section of the ion exchange membrane. That is,the average distance (d1) and the average distance (d3) in at least onecross section selected from the MD cross section and the TD crosssection preferably satisfy the relations of the above formulas.

In the present invention, it is preferred that an average value of theaverage distance (d1) in the MD cross section and the average distance(d1) in the TD cross section, and an average value of the averagedistance (d3) in the MD cross section and the average distance (d3) inthe TD cross section, satisfy the above formulas, and it is morepreferred that in both the MD cross section and the TD cross section,the average distance (d1) and the average distance (d3) satisfy therelations of the above formulas.

The values of the average distance (d1) and the average distance (d3) inthe MD cross section are obtainable by measuring, in the MD crosssection of the ion exchange membrane, the average distance (d1) and theaverage distance (d3) at 10 locations randomly selected, and obtainingthe respective average values. The average distance (d1) and the averagedistance (d3) in the TD cross section are obtainable also in the samemanner.

In the present invention, in a cross section perpendicular to the lengthdirection of the reinforcing yarns of the ion exchange membrane, thedistance (d3′) from the center of a reinforcing yarn to the center ofthe adjacent elution hole, preferably satisfies the relation of thefollowing formula (2′), more preferably satisfies the relation of thefollowing formula (2′-1), further preferably satisfies the relation ofthe following formula (2′-2), at all of the measurement points. Thereby,it becomes easy to obtain the effect of reducing the electrolysisvoltage during the alkali chloride electrolysis, while increasing themembrane strength. Here, in the distance d3′, all of the measurementpoints measured to determine the average distance (d3) mean all of themeasurement points measured to calculate the average distance (d3).Specifically, in an optional MD cross section or TD cross section, theymean measurement points at 10 locations measured to obtain the averagedistance (d3).1.0≤{d3′/d1×(n+1)}≤2.0  (2′)1.2≤{d3′/d1×(n+1)}≤1.8  (2′-1)1.5≤{d3′/d1×(n+1)}≤1.8  (2′-2)provided that the symbols in the formula (2′) have the followingmeanings,

d3′: the distance from the center of an elution hole to the center ofthe adjacent elution hole,

d1 and n: the same as above.

[Production Method]

The ion exchange membrane 1 in the present invention is preferablyproduced via the above step (i), and the step (i) preferably comprisesthe following steps (a) and (b).

Step (a): a step of obtaining a reinforcing precursor membrane byembedding a reinforcing fabric made of reinforcing yarns and sacrificialyarns, in a fluoropolymer having groups convertible to ion exchangegroups,

Step (b): a step of contacting the reinforcing precursor membraneobtained in the step (a) to an alkaline aqueous solution to convert thefluoropolymer having groups convertible to ion exchange groups, to afluoropolymer having ion exchange groups and at the same time, to let atleast a portion of the sacrificial yarns of the embedded reinforcingfabric, elute, thereby to obtain an ion exchange membrane 1 comprising afluoropolymer having ion exchange groups, a reinforcing material havingat least a portion of the sacrificial yarns in the reinforcing fabriceluted, and elution holes.

Here, in the step (b), after converting the groups convertible to ionexchange groups, to ion exchange groups, if necessary, salt exchange toreplace the counter cation of the ion exchange groups may be carriedout. In the salt exchange, the counter cation of the ion exchange groupmay be replaced, for example, from potassium to sodium.

(Step (a))

In the step (a), firstly, by a co-extrusion method, a laminated membranecomprising a layer (hereinafter referred to as “layer (C′)”) made of afluoropolymer having groups convertible to carboxylic acid functionalgroups, and a layer (hereinafter referred to as “layer (S′)”) made of afluoropolymer having groups convertible to sulfonic acid functionalgroups, is obtained. Further, separately, by a single layer extrusionprocess, a membrane (hereinafter referred to as “membrane (S′)”) made ofa layer (S′) of a fluoropolymer having groups convertible to sulfonicacid functional groups, is obtained.

Then, the membrane (S′), a reinforcing fabric and the laminated membraneare disposed in this order and laminated by means of a laminating rollor vacuum laminating apparatus. At that time, the laminated membrane ofthe laminated membrane (S′) and the layer (C′) is disposed so that thelayer (S′) is in contact with the reinforcing fabric.

(Fluoromonomers Having Groups Convertible to Carboxylic Acid FunctionalGroups)

The fluoropolymer (C′) to form the layer (C′) may, for example, be acopolymer having units derived from a fluoromonomer (hereinafterreferred to as “a fluoromonomer (C′)”) having a group convertible to acarboxylic acid functional group and units derived from a fluorinatedolefin.

The fluoromonomer (C′) is not particularly limited so long as it is acompound having one or more fluorine atoms in the molecule, an ethylenicdouble bond, and a group convertible to a carboxylic acid functionalgroup, and it is possible to use a conventional one.

As the fluoromonomer (C′), a fluorovinyl ether represented by thefollowing formula (3) is preferred from the viewpoint of the productioncost of the monomer, the reactivity with other monomers and excellentproperties of the obtainable fluoropolymer.CF₂═CF—(O)_(p)—(CF₂)_(q)—(CF₂CFX)_(r)—(O)_(s)—(CF₂)_(t)—(CF₂CFX′)_(u)-A¹  (3)

In the formula (3), X is a fluorine atom or a trifluoromethyl group. X′is a fluorine atom or a trifluoromethyl group. X and X′ in the formula(3) may be the same or different.

A¹ is a group convertible to a carboxylic acid functional group. Thegroup convertible to a carboxylic acid functional group is a functionalgroup which can be converted to a carboxylic acid functional group byhydrolysis. As the group convertible to a carboxylic acid functionalgroup, for example, —CN, —COF, —COOR¹ (wherein R¹ is a C₁₋₁₀ alkylgroup), —COONR²R³ (wherein R² and R³ are each a hydrogen atom or a C₁₋₁₀alkyl group, and R² and R³ may be the same or different), etc., may bementioned.

p is 0 or 1, q is an integer from 0 to 12, r is an integer of from 0 to3, s is 0 or 1, t is an integer from 0 to 12, and u is an integer offrom 0 to 3. However, p and s are not 0 at the same time, and r and uare not 0 at the same time. That is, 1≤p+s, and 1≤r+u.

As specific examples of the fluorovinyl ether of the formula (3), thefollowing compounds may be mentioned, and, from the viewpoint of easyproduction, a compound wherein p=1, q=0, r=1, s=0 to 1, t=1 to 3 and u=0to 1, is preferred.

CF₂═CF—O—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂CF₂—CF₂—COOCH₃,

CF₂═CF—O—CF₂CF₂—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂CF₂—O—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂CF₂—O—CF₂CF₂—CF₂—COOCH₃,

CF₂═CF—O—CF₂CF₂—O—CF₂CF₂—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂—CF₂CF₂—O—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—COOCH₃,

CF₂═CF—O—CF₂CF(CF₃)—O—CF₂—CF₂CF₂—COOCH₂.

As the fluoromonomer (C′), one type may be used alone, or two or moretypes may be used in combination.

The fluorinated olefin may, for example, be a C₂₋₃ fluoroolefin havingone or more fluorine atoms in the molecule. As such a fluoroolefin,tetrafluoroethylene (CF₂═CF₂) (hereinafter referred to as TFE),chlorotrifluoroethylene (CF₂═CFCl), vinylidene fluoride (CF₂═CH₂), vinylfluoride (CH₂═CHF), hexafluoropropylene (CF₂═CFCF₃), etc. may bementioned. Among them, TFE is particularly preferred from the viewpointof the production cost of the monomer, the reactivity with othermonomers and excellent properties of the obtainable fluoropolymer.

As the fluorinated olefin, one type may be used alone, or two or moretypes may be used in combination.

For the production of the fluoropolymer (C′) to form the layer (C′), inaddition to the fluoromonomer (C′) having a group convertible to acarboxylic acid functional group and the fluorinated olefin, othermonomers may further be used. Such other monomers may, for example, beCF₂═CF—R^(f) (wherein R^(f) is a perfluoroalkyl group having from 2 to10 carbon atoms), CF₂═CF—OR^(f1) (wherein R^(f1) is a perfluoroalkylgroup having from 1 to 10 carbon atoms), CF₂═CFO(CF₂)_(v)CF═CF₂ (whereinv is an integer of from 1 to 3), etc. By copolymerizing other monomers,it is possible to improve flexibility and mechanical strength of the ionexchange membrane 1. The proportion of other monomers is preferably atmost 30 mass % in the total monomers (100 mass %), from the viewpoint ofmaintaining the ion exchange performance.

The ion exchange capacity of the fluoropolymer (C) is preferably from0.5 to 2.0 meq/g dry resin. The ion exchange capacity of thefluoromonomer (C′) having a group convertible to a carboxylic acidfunctional group is preferably at least 0.6 meq/g dry resin, morepreferably at least 0.7 meq/g dry resin, from the viewpoint ofmechanical strength and electrochemical performance as an ion exchangemembrane.

In order to bring the ion exchange capacity of the fluoropolymer (C′) tobe within the above range, the content of units derived from afluoromonomer (C′) in the fluoropolymer (C′) may be made so that theion-exchange capacity of the fluoropolymer (C′) will be within the aboverange after converting the groups to be convertible to carboxylic acidfunctional groups in the fluoropolymer (C′) to carboxylic acidfunctional groups. The content of carboxylic acid functional groups inthe fluoropolymer (C′) is preferably the same as the content of thegroups convertible to carboxylic acid functional groups in thefluoropolymer (C′).

With respect to the molecular weight of the fluoropolymer (C′), from theviewpoint of mechanical strength and film-forming ability as an ionexchange membrane, the TQ value is preferably 150° C., more preferablyfrom 170 to 340° C., further preferably from 170 to 300° C.

The TQ value is a value related to the molecular weight of a polymer,and is one represented by a temperature showing a volume flow rate: 100mm³/sec. The volume flow rate is one obtained by letting a polymer bemelted and flow out from an orifice (diameter: 1 mm, length: 1 mm) at aconstant temperature under a pressure of 3 MPa and representing theamount of the flowing out polymer by a unit of mm³/sec. The TQ valueserves as an index for the molecular weight of the polymer and indicatesthat the higher the TQ value, the higher the molecular weight.

(Fluoropolymers Having Groups Convertible to Sulfonic Acid FunctionalGroups)

The fluoropolymer (S′) to form the layer (S′) may, for example, be acopolymer having units derived from a fluoromonomer having a groupconvertible to a sulfonic acid functional group (hereinafter referred toas “a fluoromonomer (S′)” and units derived from a fluorinated olefin.

The fluoromonomer (S′) is not particularly limited so long as it has oneor more fluorine atoms in the molecule, an ethylenic double bond and agroup convertible to a sulfonic acid functional group, and aconventional one may be employed.

As the fluoromonomer (S′), from the viewpoint of the production cost ofthe monomer, the reactivity with other monomers and excellent propertiesof the obtainable fluoropolymer, a compound represented by the followingformula (4) or a compound represented by the following formula (5) ispreferred.CF₂═CF—O—R^(f2)-A²  (4),CF₂═CF—R^(f2)-A²  (5).

R^(f2) is a perfluoroalkylene group having from 1 to 20 carbon atoms,may contain an etheric oxygen atom, and may be straight-chained orbranched.

A² is a group convertible to a sulfonic acid functional group. The groupconvertible to a sulfonic acid functional group is a functional groupthat can be converted to a sulfonic acid functional group by hydrolysis.The functional group that can be converted to a sulfonic acid functionalgroup may, for example, be —SO₂F, —SO₂Cl, —SO₂Br, etc.

As the compound represented by the formula (4), the following compoundsare specifically preferred.

CF₂═CF—O—(CF₂)_(a)—SO₂F (wherein a is an integer of from 1 to 8),

CF₂═CF—O—CF₂CF(CF₃)O(CF₂)_(a)—SO₂F (wherein a is an integer of from 1 to8),

CF₂═CF[OCF₂CF(CF₃)]_(a)SO₂F (wherein a is an integer of from 1 to 5).

As the compound represented by the formula (5), the following compoundsare specifically preferred.

CF₂═CF(CF₂)_(b)—SO₂F (wherein b is an integer of from 1 to 8),

CF₂═CF—CF₂—O—(CF₂)_(b)—SO₂F (wherein b is an integer of from 1 to 8).

As the fluoromonomer (S′), from such a viewpoint that industrialsynthesis is easy, the following compounds are more preferred.

CF₂═CFOCF₂CF₂SO₂F,

CF₂═CFOCF₂CF₂CF₂SO₂F,

CF₂═CFOCF₂CF₂CF₂CF₂SO₂F,

CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F,

CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₂SO₂F,

CF₂═CFOCF₂CF(CF₃)SO₂F,

CF₂═CFCF₂CF₂SO₂F,

CF₂═CFCF₂CF₂CF₂SO₂F,

CF₂═CF—CF₂—O—CF₂CF₂—SO₂F.

As the fluoromonomer (S′), one type may be used alone, or two or moretypes may be used in combination.

The fluorinated olefin may be those exemplified above, and from theviewpoint of the production cost of the monomer, the reactivity withother monomers and excellent properties of the obtainable fluoropolymer,TFE is particularly preferred.

As the fluorinated olefin, one type may be used alone, or two or moretypes may be used in combination.

For the production of the fluoropolymer (S′) to form the layer (S′), inaddition to the fluoromonomer (S′) and the fluorinated olefin, othermonomers may further be used.

Such other monomers may be those exemplified above. By copolymerizingother monomers, it is possible to improve flexibility and mechanicalstrength of the ion exchange membrane 1. The proportion of othermonomers is preferably at most 30 mass % in all monomers (100 mass %),from the viewpoint of maintaining the ion exchange performance.

The ion exchange capacity of the fluoropolymer (S′) is preferably from0.5 to 2.0 meq/g dry resin. The ion exchange capacity of thefluoropolymer (S′) is preferably at least 0.6 meq/g dry resin, morepreferably at least 0.7 meq/g dry resin, from the viewpoint ofmechanical strength and electrochemical performance as an ion exchangemembrane.

In order to bring the ion exchange capacity of the fluoropolymer (S) tobe within the above range, the content of units derived from afluoromonomer (S′) in the fluoropolymer (S′) may be adjusted so that theion exchange capacity of the fluoropolymer (S) will be within the aboverange, after converting the groups convertible to sulfonic acidfunctional groups in the fluoropolymer (S′) to sulfonic acid functionalgroups. The content of sulfonic acid functional groups in thefluoropolymer (S) is preferably the same as the content of the groupsconvertible to sulfonic acid functional groups in the fluoropolymer(S′).

With respect to the molecular weight of the fluoropolymer (S′), from theviewpoint of mechanical strength and membrane-forming ability as an ionexchange membrane, the TQ value is preferably at least 150° C., morepreferably from 170 to 340° C., further preferably from 170 to 300° C.

(Step (b))

An ion exchange membrane 1 is obtained by convert at least a portion ofthe groups convertible to carboxylic acid functional groups and thegroups convertible to sulfonic acid functional groups in the reinforcingprecursor membrane obtained in the above step (a), to carboxylic acidfunctional groups and sulfonic acid functional groups, respectively. Themethod for hydrolysis may, for example, be preferably a method of usinga mixture of a water-soluble organic compound and a hydroxide of analkali metal, as described in e.g. JP-A-H03-6240.

In the step (b), by contacting the reinforcing precursor membrane to analkaline aqueous solution, at least a portion of sacrificial yarns 24 iseluted by hydrolysis in the alkaline aqueous solution. The elution ofsacrificial yarns 24 is preferably carried out by hydrolysis of thematerial constituting the sacrificial yarns.

[Advantageous Effects]

In a case where an ion exchange membrane is reinforced by a reinforcingmaterial formed of reinforcing yarns and optionally containedsacrificial yarns, it is considered that the reinforcing yarns willprevent migration of cations such as sodium ions in the membrane,whereby the vicinity on the cathode side of the reinforcing yarns willbe a region (hereinafter referred to as a current-shielding region) notsubstantially functioning as an electrolysis site. Therefore, if thedensity of the reinforcing yarns is increased by narrowing theirspacing, the current-shielding region within the ion exchange membranewill be increased, whereby it is considered the membrane resistance willincrease, and the electrolysis voltage becomes high.

Whereas, in the ion exchange membrane for alkali chloride electrolysisof the present invention, the above-mentioned average distance (d1), thetotal area (S) and the number (n) of elution holes, are controlled to bewithin the specific ranges, whereby even if the membrane strength isincreased by narrowing the spacing of reinforcing yarns, the membraneresistance is kept low, and it is possible to reduce the electrolysisvoltage at the time of alkali chloride electrolysis. This is consideredto be as follows.

When the total area (S) is small, sodium ions, etc. tend to be difficultto pass through portions of elution holes in the vicinity of thereinforcing yarns, whereby the membrane resistance in the vicinity ofthe reinforcing yarns tends to be higher than when the total area (S) islarge. On the other hand, at a portion apart from reinforcing yarns, thevolume of elution holes is small as compared to when the total area (S)is large, whereby extra resistance will not increase, and the membraneresistance tends to be low. Further, when the above total area (S) islarge, sodium ions, etc. tend to easily pass together with the saltwater through elution holes in the vicinity of reinforcing yarns, andthe current shielding region becomes smaller, whereby the membraneresistance in the vicinity of reinforcing yarns becomes low as comparedto when the total area (S) is small. On the other hand, at a portionapart from reinforcing yarns, the volume of elution holes is larger thanwhen the total area (S) is small, whereby extra resistance tends toincrease, and the membrane resistance tends to increase.

Further, when the number (n) of elution holes is small, like in the casewhere the total area (S) is small, sodium ions, etc. tend to bedifficult to pass in the vicinity of reinforcing yarns, and the membraneresistance in the vicinity of reinforcing yarns tend to be high ascompared to when the number (n) of elution holes is large. On the otherhand, at a portion apart from reinforcing yarns, the volume of elutionholes is small, whereby extra resistance will not increase, and themembrane resistance tends to be low as compared to when the number (n)of elution holes is large. Further, when the above-described number (n)of elution holes is large, sodium ions, etc. tend to easily pass in thevicinity of reinforcing yarns, and the current shielding region becomessmaller, whereby the membrane resistance in the vicinity of reinforcingyarns tends to be low as compared to when the number n of elution holesis small. On the other hand, at a portion apart from reinforcing yarns,the volume of elution holes increases, whereby extra resistance willincrease, and the membrane resistance becomes high as compared to whenthe number n of elution holes is small. From the foregoing, it isimportant to control the number (n) of elution holes and the total area(S) to be within the specific range.

If the spacing of reinforcing yarns is made to be too narrow and thecurrent shielded region within the ion exchange membrane becomesextremely large, the effects to reduce the membrane resistance by thetotal area (S) and the number (n) of elution holes tend to be relativelysmall. On the other hand, if the spacing of the reinforcing yarns ismade to be too wide, and the current shielding region becomes extremelysmall, elution holes tend to be extra resistance, and the effect toreduce the electrolysis voltage tends to be relatively small. Therefore,it is important to control the spacing of reinforcing yarns to be withinthe specific range.

In the present invention, by controlling the above-described total area(S) and number n of elution holes within specific ranges, the currentshielding region in the vicinity of reinforcing yarns is made small tolower the membrane resistance in the vicinity of reinforcing yarns, andat the same time, the volume of elution holes at a portion apart fromreinforcing yarns is maintained to be small to some extent, wherebyincrease in the membrane resistance at this portion is suppressed. Thus,as compared with the degree of increase in membrane resistance at aportion apart from reinforcing yarns, the degree of decrease in themembrane resistance in the vicinity of reinforcing yarns becomes large,whereby it is considered that, as the entire membrane, the membraneresistance becomes low, and even if the spacing of reinforcing yarns ismade narrow to increase the membrane strength, it is possible to reducethe electrolysis voltage at the time of alkali chloride electrolysis.

[Other Embodiments]

Further, the ion exchange membrane of the present invention is notlimited to the ion-exchange membrane 1 as described above.

For example, the ion exchange membrane of the present invention may beone wherein the electrolyte membrane is a membrane of a single layer, ora laminate having layers other than the layer (C) and the layer (S).Further, it may be one wherein the reinforcing fabric is embedded in thelayer (C).

Further, the sacrificial yarns are not limited to monofilaments asillustrated in the drawings, and may be multifilaments.

<Alkali Chloride Electrolysis Apparatus>

For the alkali chloride electrolysis apparatus of the present invention,a known embodiment may be employed except for using the ion exchangemembrane for alkali chloride electrolysis of the present invention. FIG.3 is a schematic diagram showing an example of the alkali chlorideelectrolysis apparatus of the present invention.

The alkali chloride electrolysis apparatus 100 of this embodimentcomprises an electrolytic cell 110 provided with a cathode 112 and ananode 114, and an ion exchange membrane 1 installed in the electrolyticcell 110 so as to partition inside of the electrolytic cell 110 into acathode chamber 116 on the cathode 112 side and an anode chamber 118 onthe anode 114 side.

The ion exchange membrane 1 is installed in the electrolytic cell 110 sothat the layer (C) 12 is located on the cathode 112 side, and the layer(S) 14 is located on the anode 114 side.

The cathode 112 may be disposed in contact with the ion exchangemembrane 1 or may be disposed with a space from the ion exchangemembrane 1.

As the material constituting the cathode chamber 116, preferred is amaterial which is resistant to sodium hydroxide and hydrogen. As such amaterial, stainless steel, nickel, etc. may be mentioned.

As the material constituting the anode chamber 118, preferred is amaterial which is resistant to sodium chloride and chlorine. As such amaterial, titanium may be mentioned.

For example, in a case where an aqueous solution of sodium hydroxide isto be produced by electrolysis of a potassium chloride aqueous solution,by supplying a sodium chloride aqueous solution to the anode chamber 118of the alkali chloride electrolysis apparatus 100, and supplying asodium hydroxide aqueous solution to the cathode compartment 116, thesodium chloride aqueous solution is electrolyzed while maintaining theconcentration of the sodium hydroxide aqueous solution discharged fromthe cathode chamber 116 at a predetermined concentration (e.g. 32 mass%).

According to the alkali chloride electrolysis apparatus of the presentinvention as described above has the ion exchange membrane for alkalichloride electrolysis of the present invention, and thus the membranestrength is high, and it is possible to reduce the electrolysis voltageduring the alkaline chloride electrolysis.

EXAMPLES

In the following, the present invention will be described in detail withreference to Examples, but the present invention is not limited by theseExamples. Ex. 1 to 2, 5 and 8 are Examples of the present invention, andEx. 3, 4, 6 and 7 are Comparative Examples.

[Measurement Method of TQ Value]

The TQ value is a value related to the molecular weight of a polymer andwas obtained as a temperature showing a volume flow rate of 100 mm³/sec.The volume flow rate is a flow out amount (unit: mm³/sec.) when afluoropolymer having groups convertible to ion exchange groups, ismelted and permitted to flow out from an orifice (diameter: 1 mm,length: 1 mm) at a constant temperature under a pressure of 3 MPa, byusing Shimadzu Flow Tester CFD-100D (manufactured by ShimadzuCorporation).

[Measurement Method of Ion Exchange Capacity]

About 0.5 g of a fluoropolymer having groups convertible to ion exchangegroups was formed into a film by flat pressing, and then, the film wasanalyzed by a transmission infrared spectroscopy apparatus. The ratio ofthe units having groups convertible to carboxylic acid functional groupsor the units having groups convertible to sulfonic acid functionalgroups was calculated by using the respective peak heights of CF₂ peak,CF₃ peak, OH peak, CF peak and SO₂F peak, of the obtained spectra, andthis was regarded as the ratio of the units having carboxylic acidfunctional groups or the units having sulfonic acid functional groups inthe fluoropolymer obtained after hydrolysis treatment. Then, the ionexchange capacity was calculated by using known samples as thecalibration curve.

Further, a film having an ion exchange group whose terminal group isacid type or K type or N type can be measured similarly.

[Measurement Method of Distance of Reinforcing Yarns and Elution Holes]

By observing a cross section of reinforcing yarns, of an ion exchangemembrane dried at 90° C. for at least 2 hours in the atmosphere by anoptical microscope, the distance was measured by using an image software(Pixs2000 PRO manufactured by INNOTECH CORPORATION). In the measurement,in each of the MD cross section and the TD cross section, the distancefrom the center of a reinforcing yarn to the center of the adjacentreinforcing yarn was measured at 10 points. With respect to the MD crosssection, the average distance (d1) being an average value of themeasured values at 10 points, was obtained. Also with respect to the TDcross section, the average distance (d1) was obtained in the samemanner.

Further, the average distances (d2) and (d3) were obtained in the samemanner. The average distance d1 was obtained from the average. Theaverage values d2 and d3 were obtained in the same manner.

Here, the average distances (d1), (d2) and (d3) are values of areinforcing material embedded in an ion exchange membrane produced viathe steps (a) and (b).

[Measurement Method of Cross-Sectional Area]

By observing a cross section cut perpendicular to the length directionof reinforcing yarns, of an ion exchange membrane dried at 90° C. for atleast 2 hours in the atmosphere by an optical microscope, the total area(S) obtained by adding the cross-sectional area of an elution hole andthe cross-sectional area of a sacrificial yarn, was measured by using animage software (Pixs2000 PRO manufactured by INNOTECH CORPORATION). Thetotal area (S) was measured at 10 points randomly selected in each ofthe MD cross section and the TD cross section. With respect to the MDcross section, the total area (S) was obtained as an average value ofthe measured values at 10 points. Also with respect to the TD crosssection, the total area (S) was obtained in the same manner.

In a case where a sacrificial yarn is completely dissolved, the totalarea (S) is the cross-sectional area of the elution hole, and in a casewhere a sacrificial yarn remaining undissolved is present in an elutionhole, the total area (S) is a value obtained by adding thecross-sectional area of the elution hole and the cross-sectional area ofthe sacrificial yarn remaining undissolved.

[Measurement Method of Width of Reinforcing Yarn]

By observing a cross section of an ion exchange membrane dried at 90° C.for at least 2 hours in the atmosphere by an optical microscope, thewidth of a reinforcing fabric as viewed in a direction perpendicular tothe fabric surface of a reinforcing material was measured by using animage software (Pixs2000 PRO manufactured by INNOTECH CORPORATION). Thewidth of the reinforcing yarn was measured at 10 points in each of theMD cross section and the TD cross section, and then the average valuewas obtained.

[Measurement of Aperture Ratio]

For the aperture ratio, by observing a cross section cut perpendicularto the length direction of reinforcing yarns, of an ion exchangemembrane dried at 90° C. for at least 2 hours in the atmosphere by anoptical microscope, the aperture ratio was calculated by using an imagesoftware (Pixs2000 PRO manufactured by INNOTECH CORPORATION). For thecalculation, in each of the MD cross section (cross section cutperpendicular to the MD direction) and the TD cross section (crosssection cut perpendicular to the TD direction), the distance from thecenter of a reinforcing yarn to the center of the adjacent reinforcingyarn, and the width of a reinforcing yarn were measured at 10 points,and the aperture ratio was calculated from the following formula.{(Distance between reinforcing yarns in the MD cross section−width ofreinforcing yarn in the MD cross section)×(distance between reinforcingyarns in the TD cross section−width of reinforcing yarn in the TD crosssection)}/{(distance between reinforcing yarns in the MD crosssection)×(distance between reinforcing yarns in TD cross section)}×100[Measurement Method of Electrolysis Voltage]

The ion exchange membrane was installed in a test electrolytic cell withan electrolytic surface size of 150 mm×100 mm so that the layer (C)faced the cathode. DSE manufactured by PERMELEC ELECTRODE LTD. was usedas the anode, and Raney nickel plating cathode manufactured by CHLORINEENGINEERS was used as the cathode. Electrolysis of a sodium chlorideaqueous solution was conducted under conditions of a sodium hydroxideconcentration of 32 mass %, a sodium chloride concentration of 200 g/L,a temperature of 90° C. and a current density of 8 kA/m², whereby theelectrolysis voltage (V) was measured after 3 to 10 days from theinitiation of operation.

[Ex. 1]

TFE and a fluoromonomer having groups convertible to carboxylic acidfunctional groups, represented by the following formula (3-1) werecopolymerized to synthesize a fluoropolymer having groups convertible tocarboxylic acid functional groups (ion exchange capacity: 1.06 meq/g dryresin, TO: 225° C.) (hereinafter referred to as polymer C).CF₂═CF—O—CF₂CF₂—CF₂—COOCH₃  (3-1).

TFE and a fluoromonomer having groups convertible to sulfonic acidfunctional groups, represented by the following formula (4-1) werecopolymerized to synthesize a fluoropolymer having groups convertible tosulfonic acid functional groups (ion exchange capacity: 1.1 meq/g dryresin, TQ: 235° C.) (hereinafter referred to as polymer S).CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F  (4-1).

The polymer C and the polymer S were molded by a co-extrusion method toobtain a film A of a two layer structure with a layer (C′) (thickness:12 μm) made of the polymer C and a layer (S′) (thickness: 68 μm) made ofthe polymer S.

Further, the polymer S was molded by a melt extrusion method to obtain afilm B comprising a layer (S′) (thickness: 30 μm).

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 2,000 times/m toobtain a PTFE yarn, which was used as a reinforcing yarn. A PET yarnmade of a monofilament of 7 denier, was used as a sacrificial yarn.Plain weaving was conducted so that one reinforcing yarn and 10sacrificial yarns would be alternately arranged, to obtain a reinforcingfabric (density of reinforcing yarns: 27 yarns/inch, density ofsacrificial yarns: 270 yarns/inch).

The film B, the reinforcing fabric, the film A and a release PET film(thickness: 100 μm) were overlaid in this order so that the layer (C′)of the film A was located on the release PET film side, and laminated byusing a roll. The release PET film was peeled off to obtain areinforcing precursor membrane.

A paste comprising 29.0 mass % of zirconium oxide (average particlediameter: 1 μm), 1.3 mass % of methyl cellulose, 4.6 mass % ofcyclohexanol, 1.5 mass % of cyclohexane and 63.6 mass % of water, wastransferred by a roll press on the layer (S′) side of the reinforcingprecursor membrane, to form a gas-releasing coating layer. The attachedamount of zirconium oxide was 20 g/m².

The reinforcing precursor film having the gas-releasing coating layerformed on one side, was immersed in an aqueous solution containing 5mass % of dimethyl sulfoxide and 30 mass % of potassium hydroxide at 95°C. for 8 minutes. Thus, —COOCH₃ of the polymer C and —SO₂F of thepolymer S were hydrolyzed and converted to ion exchange groups, toobtain a membrane having the layer (C′) as the layer (C) and the layer(S′) as the layer (S).

In an ethanol solution containing 2.5 mass % of an acid-form polymer ofpolymer S, zirconium oxide (average particle diameter: 1 μm) wasdispersed at a concentration of 13 mass %, to prepare a dispersion. Thedispersion was sprayed on the layer (C) side of the membrane, to form agas releasing coating layer, to obtain an ion exchange membrane havinggas releasing coating layers formed on both surfaces. The attachedamount of zirconium oxide was 3 g/m².

[Ex. 2]

An ion exchange membrane was obtained in the same manner as in Ex. 1,except that as the sacrificial yarn, a PET yarn made of a monofilamentof 7 denier was used and plain weaving was conducted so that onereinforcing yarn and 12 sacrificial yarns would be alternately arranged,to obtain a reinforcing fabric (density of sacrificial yarns: 324yarns/inch).

[Ex. 3]

An ion exchange membrane was obtained in the same manner as in Ex. 1,except that as the sacrificial yarn, a PET yarn made of a monofilamentof 9 denier was used and plain weaving was conducted so that onereinforcing yarn and four sacrificial yarns would be alternatelyarranged, to obtain a reinforcing fabric (density of sacrificial yarns:108 yarns/inch).

[Ex. 4]

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 450 times/m to obtaina PTFE yarn, which was used as a reinforcing yarn. As the sacrificialyarn, a PET yarn made of a monofilament of 12 denier was used. Plainweaving was conducted so that one reinforcing yarn and eight sacrificialyarns would be alternately arranged, to obtain a reinforcing fabric(density of reinforcing yarns: 20 yarns/inch, density of sacrificialyarns: 160 yarns/inch). Other than these, an ion exchange membrane wasobtained in the same manner as in Ex. 1.

[Ex. 5]

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 450 times/m to obtaina PTFE yarn, which was used as a reinforcing yarn. As the sacrificialyarn, a PET yarn made of a monofilament of 12 denier was used. Plainweaving was conducted so that one reinforcing yarn and 10 sacrificialyarns would be alternately arranged, to obtain a reinforcing fabric(density of reinforcing yarns: 20 yarns/inch, density of sacrificialyarns: 200 yarns/inch). Other than these, an ion exchange membrane wasobtained in the same manner as in Ex. 1.

[Ex. 6]

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 450 times/m to obtaina PTFE yarn, which was used as a reinforcing yarn. As the sacrificialyarn, a PET yarn made of a monofilament of 5 denier was used. Plainweaving was conducted so that one reinforcing yarn and 10 sacrificialyarns would be alternately arranged, to obtain a reinforcing fabric(density of reinforcing yarns: 20 yarns/inch, density of sacrificialyarns: 200 yarns/inch). Other than these, an ion exchange membrane wasobtained in the same manner as in Ex. 1.

[Ex. 7]

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 450 times/m to obtaina PTFE yarn, which was used as a reinforcing yarn. As the sacrificialyarn, a PET yarn made of a multifilament of 3.3 denier was used. Plainweaving was conducted so that one reinforcing yarn and 10 sacrificialyarns would be alternately arranged, to obtain a reinforcing fabric(density of reinforcing yarns: 20 yarns/inch, density of sacrificialyarns: 200 yarns/inch). Other than these, an ion exchange membrane wasobtained in the same manner as in Ex. 1.

[Ex. 8]

A PTFE film was rapidly stretched and then slit to a thickness of 100denier to obtain a monofilament, which was twisted 450 times/m to obtaina PTFE yarn, which was used as a reinforcing yarn. As the sacrificialyarn, a PET yarn made of a monofilament of 12 denier was used. Plainweaving was conducted so that one reinforcing yarn and 10 sacrificialyarns would be alternately arranged, to obtain a reinforcing fabric(density of reinforcing yarns: 17 yarns/inch, density of sacrificialyarns: 170 yarns/inch). Other than these, an ion exchange membrane wasobtained in the same manner as in Ex. 1.

The results of measurements of the average distances (d1), (d2) and(d3), the total area (S), the width of reinforcing yarn, and theelectrolysis voltage, of the ion exchange membrane in each Ex., areshown in Table 1.

TABLE 1 Unit Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Rein-Fineness Denier 100 100 100 100 100 100 100 100 forcing Filament Number1 1 1 1 1 1 1 1 yarn Density of Number/ 27 27 27 20 20 20 20 17 yarnsinch Sacri- Fineness Denier 7 7 9 12 12 5 20 12 ficial Filament Number 11 1 1 1 1 6 1 yarn Density of Number/ 270 324 108 160 200 200 200 170yarns inch Weaving — Possible Possible Possible Possible Possible NotPossible Possible (possible/not possible) possible MD d1 μm 908 976 8531150 1125 — 1250 1450 cross S μm² 651 620 726 980 999 — 1900 950 sectionn Number 10 12 4 8 10 — 10 10 d2 μm 70 68 147 120 86 — 93 110 d2/d1 ×(n + 1) — 0.85 0.91 0.86 0.94 0.84 — 0.82 0.83 d3 μm 139 114 206 155 176— 207 230 d3/d1 × (n + 1) — 1.68 1.52 1.21 1.21 1.72 — 1.82 1.74 Widthof rein- μm 128 129 110 145 147 — 148 143 forcing yarn TD d1 μm 910 929850 1152 1135 — 1220 1480 cross S μm² 680 590 701 1021 1000 — 1935 900section n Number 10 12 4 8 10 — 10 10 d2 μm 70 66 140 115 88 — 98 108d2/d1 × (n + 1) — 0.85 0.92 0.82 0.90 0.85 — 0.88 0.80 d3 μm 140 115 215174 172 — 169 254 D3/d1 × (n + 1) — 1.69 1.61 1.26 1.36 1.66 — 1.52 1.89Width of μm 125 123 114 152 150 — 146 142 reinforcing yarn Average d1 μm909 953 852 1151 1130 — 1235 1465 of S μm² 666 605 714 1001 1000 — 1918925 MD n Number 10 12 4 8 10 — 10 10 cross d2 μm 70 67 144 118 87 — 96109 section d2/d1 × (n + 1) — 0.85 0.91 0.84 0.92 0.85 — 0.85 0.82 andd3 μm 140 115 211 164 174 — 188 242 TD d3/d1 × (n + 1) — 1.69 1.56 1.231.28 1.69 — 1.67 1.82 cross Width of μm 127 126 112 149 149 — 147 143section reinforcing yarn Aperture ratio % 75 75 75 81 82 — 81 81Electrolysis voltage V 3.28 3.26 3.30 3.30 3.27 — 3.31 3.29

As shown in Table 1, in Ex. 1 to 2 using ion exchange membranes whereinthe total area (S) was from 500 to 1,200 μm² and the number (n) of theelution holes was at least 10, the electrolysis voltage was low ascompared with Ex. 3 using an ion exchange membrane wherein the number(n) of elution holes was 4.

In Ex. 5 in which the number (n) of the elution holes was 10, theelectrolysis voltage was low as compared with Ex. 4 in which the number(n) of elution holes was 8.

In Ex. 6 in which the total area (S) was less than 500 μm², thesacrifice yarns were broken in weaving, and the reinforcing fabric couldnot be weaved.

In Ex. 7 in which the total area (S) was more than 1,200 μm², theelectrolysis voltage was high as compared with Ex. 5 in which the number(n) of elution holes was the same as those of Ex. 7. Further, theelectrolysis voltage was high as compared with Ex. 4 in which the number(n) of elution holes was 8.

In Ex. 8 using ion exchange membranes wherein the total area (S) wasfrom 500 to 1,200 μm² and the number (n) of the elution holes was 10,the electrolysis voltage was low as compared with Ex. 4 wherein thenumber (n) of elution holes was 8. Moreover, in Ex. 8, the electrolysisvoltage was high and the effect was small as compared with Ex. 5 inwhich the average distance (d1) from the center of a reinforcing yarn tothe center of the adjacent reinforcing yarn is from 750 to 1,500 μm.

INDUSTRIAL APPLICABILITY

The electrolysis apparatus having the ion exchange membrane for alkalichloride electrolysis of the present invention is widely used forproduction of chloride and sodium hydroxide or potassium hydroxide byelectrolysis of a sodium chloride aqueous solution or a potassiumchloride aqueous solution.

This application is a continuation of PCT Application No.PCT/JP2016/056488, filed on Mar. 2, 2016, which is based upon and claimsthe benefit of priority from Japanese Patent Application No. 2015-041301filed on Mar. 3, 2015. The contents of those applications areincorporated herein by reference in their entireties.

REFERENCE SYMBOLS

1: ion exchange membrane for alkali chloride electrolysis, 10:electrolyte membrane, 12: layer (C), 14: layer (S), 20: reinforcingmaterial, 22: reinforcing yarn, 24: sacrificial yarn, 26: filament, 28:elution hole, 121: aqueous sodium chloride, 122: desalted aqueous sodiumchloride, 123: aqueous sodium hydroxide solution

What is claimed is:
 1. An ion exchange membrane for alkali chlorideelectrolysis, comprising: a fluoropolymer having at least one ionexchange group, a reinforcing material embedded in the fluoropolymer andformed of reinforcing yarns and optionally comprising sacrificial yarns,and elution holes of the sacrificial yarns present between thereinforcing yarns, wherein in a cross section perpendicular to a lengthdirection of the reinforcing yarns forming the reinforcing material, atotal area (S) obtained by adding a cross-sectional area of an elutionhole and a cross-sectional area of a sacrificial yarn remaining in theelution hole is from 500 to 1,200 μm², and the number (n) of elutionholes between adjacent reinforcing yarns is from 10 to
 12. 2. The ionexchange membrane for alkali chloride electrolysis according to claim 1,wherein in the cross section perpendicular to the length direction ofthe reinforcing yarns, an average distance (d1) from the center of areinforcing yarn to the center of an adjacent reinforcing yarn is from750 to 1,500 μm.
 3. The ion exchange membrane for alkali chlorideelectrolysis according to claim 1, wherein a relationship is establishedto satisfy the following formula (1) in the cross section perpendicularto the length direction of the reinforcing yarns:0.5<{d2/d1×(n+1)}<1.5  (1), wherein d1 is an average distance from thecenter of a reinforcing yarn to the center of an adjacent reinforcingyarn, d2 is an average distance from the center of an elution hole tothe center of an adjacent elution hole, and n is the number of elutionholes between adjacent reinforcing yarns.
 4. The ion exchange membranefor alkali chloride electrolysis according to claim 3, wherein arelationship is established to satisfy the following formula (1′) at allmeasurement points measured to determine the average distance (d1) andthe average distance (d2) in the cross section perpendicular to thelength direction of the reinforcing yarns:0.5<{d2′/d1×(n+)}<1.5  (1′), wherein d2′ is a distance from the centerof an elution hole to the center of an adjacent elution hole.
 5. The ionexchange membrane for alkali chloride electrolysis according to claim 1,wherein a relationship is established to satisfy the following formula(2) in the cross section perpendicular to the length direction of thereinforcing yarns:1.0<{d3/d1×(n+1)}<2.0  (2), wherein d3 is an average distance from thecenter of a reinforcing yarn to the center of an adjacent elution hole,d1 is an average distance from the center of a reinforcing yarn to thecenter of the adjacent reinforcing yarn, and n is the number of elutionholes between adjacent reinforcing yarns.
 6. The ion exchange membranefor alkali chloride electrolysis according to claim 5, wherein arelationship is established to satisfy the following formula (2′) at allmeasurement points measured to determine the average distance (d1) andthe average distance (d3) in the cross section perpendicular to thelength direction of the reinforcing yarns:1.0<{d3′/d1×(n+1)}<2.0  (2′), wherein d3′ is a distance from the centerof an elution hole to the center of an adjacent elution hole.
 7. The ionexchange membrane for alkali chloride electrolysis according to claim 1,wherein widths of the reinforcing yarns in the cross sectionperpendicular to the length direction of the reinforcing yarns are from70 to 160 μm.
 8. The ion exchange membrane for alkali chlorideelectrolysis according to claim 1, wherein the fluoropolymer comprises afluoropolymer comprising at least one carboxylic acid functional groupand a fluoropolymer comprising at least one sulfonic acid functionalgroup, and the reinforcing material is embedded in the fluoropolymercomprising the at least one sulfonic acid functional group.
 9. An alkalichloride electrolysis apparatus comprising an electrolytic cell providedwith a cathode and an anode, and an ion exchange membrane for alkalichloride electrolysis as defined in claim 1 partitioning a cathodechamber on the cathode side and an anode chamber on the anode side inthe electrolytic cell.
 10. A method for producing an ion exchangemembrane for alkali chloride electrolysis, which comprises obtaining areinforcing precursor membrane having a reinforcing fabric composed ofreinforcing yarns and sacrificial yarns, embedded in a precursormembrane comprising a fluoropolymer having groups convertible to ionexchange groups, and then contacting the reinforcing precursor membraneto an alkaline aqueous solution to convert the groups convertible to ionexchange groups, to ion exchange groups, and at the same time to eluteat least a portion of the sacrificial yarns in the reinforcing fabric,thereby to obtain an ion exchange membrane comprising a fluoropolymerhaving ion exchange groups, a reinforcing material having at least aportion of the sacrificial yarns in the reinforcing fabric eluted, andelution holes, characterized in that in a cross section perpendicular tothe length direction of the reinforcing yarns forming the reinforcingmaterial in the ion exchange membrane, the total area (S) obtained byadding the cross-sectional area of an elution hole and thecross-sectional area of a sacrificial yarn remaining in the elutionhole, is from 500 to 1,200 m², and the number (n) of elution holesbetween adjacent reinforcing yarns is from 10 to
 12. 11. The method forproducing an ion exchange membrane for alkali chloride electrolysisaccording to claim 10, wherein in a cross section perpendicular to thelength direction of the reinforcing yarns forming the reinforcingmaterial, the average distance (d1) from the center of a reinforcingyarn to the center of the adjacent reinforcing yarn is from 750 to 1,500μm.
 12. The method for producing an ion exchange membrane for alkalichloride electrolysis according to claim 10, wherein a relationship isestablished to satisfy the following formula (1) in a cross sectionperpendicular to the length direction of the reinforcing yarns:0.5<{d2/d1×(n+1)}<1.5  (1) provided that the symbols in the formula (1)have the following meanings, d1: the average distance from the center ofa reinforcing yarn to the center of the adjacent reinforcing yarn, d2:the average distance from the center of an elution hole to the center ofthe adjacent elution hole, n: the number of elution holes betweenadjacent reinforcing yarns.
 13. The method for producing an ion exchangemembrane for alkali chloride electrolysis according to claim 12, whereina relationship is established to satisfy the following formula (1′) atall measurement points measured to determine the average distance (d2)in a cross section perpendicular to the length direction of thereinforcing yarns:0.5<{d2′/d1×(n+1)}<1.5  (1′) provided that the symbols in the formula(1′) have the following meanings, d1: the average distance from thecenter of a reinforcing yarn to the center of the adjacent reinforcingyarn, d2′: the distance from the center of an elution hole to the centerof the adjacent elution hole, n: the number of elution holes betweenadjacent reinforcing yarns.
 14. The method for producing an ion exchangemembrane for alkali chloride electrolysis according to claim 10, whereina relationship is established to satisfy the following formula (2) in across section perpendicular to the length direction of the reinforcingyarns:1.0<{d3/d1×(n+1)}<2.0  (2) provided that the symbols in the formula (2)have the following meanings, d1: the average distance from the center ofa reinforcing yarn to the center of the adjacent reinforcing yarn, d3:the average distance from the center of a reinforcing yarn to the centerof the adjacent elution hole or sacrificial yarn, n: the number ofelution holes between adjacent reinforcing yarns.
 15. The method forproducing an ion exchange membrane for alkali chloride electrolysisaccording to claim 14, wherein a relationship is established to satisfythe following formula (2′) at all measurement points measured todetermine the average distance (d3) in a cross section perpendicular tothe length direction of the reinforcing yarns:1.0<{da/d1×(n+1)}<2.0  (2′) provided that the symbols in the formula(2′) have the following meanings, d1: the average distance from thecenter of a reinforcing yarn to the center of the adjacent reinforcingyarn, d3′: the distance from the center of a reinforcing yarn to thecenter of the adjacent elution hole, n: the number of elution holesbetween adjacent reinforcing yarns.
 16. The method for producing an ionexchange membrane for alkali chloride electrolysis according to claim10, wherein the widths of said reinforcing yarns in a cross sectionperpendicular to the length direction of the reinforcing yarns are from70 to 160 μm.
 17. A method for producing an alkaline chlorideelectrolysis apparatus, characterized by obtaining an ion exchangemembrane for alkali chloride electrolysis by the method as defined inclaim 10, and then disposing the ion exchange membrane as an electrolytemembrane partitioning a cathode chamber on the cathode side and an anodechamber on the anode side in the electrolytic cell.